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After a record stay in space, astronaut Scott Kelly grew 1.5 inches due to a lack of gravity. He faces a greater risk of cancer than most earth-bound folks due to radiation exposure that’s 20 times higher in space. His bones are less dense than they used to be, and his heart is likely smaller.

“It felt like I had been up there my whole life, you know, after about the first six months,” Kelly said in a NASA video following his March 1 return from the International Space Station. “I’m definitely encouraged on our ability to go even longer.”

Kelly spent 340 days aboard the ISS on his fourth mission to space, along with Russian cosmonaut Mikhail Kornienko. Over his entire career, Kelly has spent 520 days in space. His twin brother, retired astronaut Mark Kelly, is providing data to help quantify the effect of a year in space on a person who is almost identical genetically.

Humans aren’t engineered for long-term space travel, which makes Kelly’s Year in Space mission a key component of NASA’s efforts to mitigate the harsh effects. If it doesn’t succeed, missions to Mars and any plans for supply or mining operations on the moon won’t be feasible. A round trip to Mars is expected to last more than 500 days.

Here are some of the things that happen to the body in space, which NASA needs to understand better:

The fluids in your body shift due to zero gravity. In the brain, this can cause impaired vision or even loss of sight, a problem NASA astronauts detected only in recent years as ISS stays became longer. Scott Kelly also had some vision issues. “He’s just starting to get an idea of the impact on his vision,” Mark Kelly said Friday at a NASA news conference. The fluid movement can alter eye shapes. The working theory is that genetics and a lack of certain B vitamins disposes some people to have this vision problem in zero gravity.

Some bones, muscles, and organs deteriorate. Astronauts can lose as much as 2 percent of their bone density each month, according to NASA, more than twice the amount the average adult loses annually. This is because of the lack of weight bones experience in space and differences in how the body processes calcium. Muscle soreness is a common complaint among space travelers. “Which muscles or muscle groups hurt?” a reporter asked Scott Kelly Friday. “Yeah, all of them,” he replied. The zero-G environment can also cause increased risk of kidney stones, hip and spine troubles, and slower healing times. A decrease in the size of the heart is related to changes in blood flows, but the issue still isn't well understood. To counteract some of these effects, astronauts on the ISS exercise frequently and vigorously.

You get taller: Gravity on earth compresses our spine, which doesn’t happen in space, as fluids flow between the spinal disks. But the effect isn’t permanent. Back on earth, you revert rapidly to your original height.

Immunity is suppressed: The human immune system doesn’t operate as well in space, and NASA isn’t sure why.

Space is a cancer danger. Astronauts are exposed to far more radiation than people on earth because of the shielding effect of the planet's atmosphere. NASA rigorously monitors astronauts’ career radiation levels—which means Kelly almost certainly has no future space time—but a long mission will require advanced work on how to mitigate this risk.

The isolation carries psychological effects, among the most significant and most mysterious aspects of long periods in space. Understanding and ameliorating the psychological effects of extended missions will be critical to any future human exploration of space, because NASA expects these impacts will be larger as mission lengths increase.